The invention relates to electronic time base error correction methods and arrangements for color video recording systems, particularly arrangements of this kind which use storage type delay lines as the time base compensating elements in the reproduction of the recorded color video signals.
Substantial elimination of time base error in the playback of recorded color video signals is essential to the production of a picture of commercially acceptable quality. Time base error is due to a number of causes, including speed variations of the recording medium--and, in the case of the use of magnetic tape as the recording medium, also tape-stretching, capstan irregularities, and the like. Time base error is conventionally defined as the time by which the distance between successive horizontal sync pulses departs from its nominal value, approximately 64 microseconds in the NTSC system. If the speed of the recording medium, such as a tape, is too high, the sync pulse will arrive too early and trigger scanning of a new line prematurely. Similarly, if the tape speed is too low, the sync pulse will be late in arriving and scanning of a new line will be triggered too late. Such time base error will not only lead to a distortion of the picture but, in the playback of color video recordings, the resulting phase distortion will give rise to a color imbalance of the displayed picture. While time base error can be reduced to some extent by servo controls acting on the tape drive motor, such mechanical compensation is usually insufficient to hold the time base error within acceptable limits; as a result, electronic time base error correction is needed.
The present invention relates more specifically to electronic time base error correction in color video recording systems in which the luminance component and the chrominance component are recorded on the recording medium in different frequency bands. In a system visualized herein by way of example, the chrominance information of the recorded signal may be accommodated in a frequency range lower than that of the luminance information of that signal; systems of this kind accordingly have become known in the art as "color-under" systems. More particularly, in a specific linear video recording system of this kind to which the invention is applicable, the chroma information may be recorded as amplitude modulated on a carrier of approximately 500 khz and the luminance information recorded as frequency modulated on a carrier of about 4 mhz. In order to reproduce correctly the chroma portion this approximately 500 khz modulated chroma signal must be synchronously demodulated after retrieval from the tape. Loss of synchronization would result in improper color reproduction. In order to insure synchronous demodulation of the chroma signal the residual time base error at the input of the chroma demodulator should preferably be less than 150 nanoseconds. Since the magnitude of time base error off-tape is normally in the order of 4 to 6 microseconds it can be seen that significant time base correction must be effected.
In systems in which the luminance portion and the chrominance portion are recorded on the tape in different frequency bands the off-tape signal, in the playback circuitry, is usually split into two separate parallel channels, a luminance channel and a chrominance channel. In an electronic time base error correction arrangement for a reproduction system of this kind an analogue delay line, for example of the storage type, is interposed in each of the two channels and the charge transfer rate of these two delay lines is controlled, in a compensating sense, with the aid of a voltage controlled oscillator under the control of a comparator in which a synchronizing signal subject to time base error--typically a sync signal separately recorded on a control track of the tape--is compared with a reference signal. This known technique, however, does not meet the exacting correction requirements present in linear video recording systems such as those outlined above.